The present invention relates to a brake system provided for motor vehicles, comprising a pedal-actuated hydraulic braking pressure generator connected to wheel brakes by way of pressure fluid lines. An auxiliary pressure supply system is provided comprising an auxiliary pressure source and an auxiliary pressure control valve causing an auxiliary pressure proportional to the pedal force. Pressure-controlled multidirectional valves are inserted into the pressure fluid conduits between the braking pressure generator and the wheel brakes, which multidirectional conduits, in their inactive or initial position, establish a hydraulic connection between the braking pressure generator and the wheel brakes and which, after change-over into a second switch position, connect the auxiliary pressure control valve in place of the braking pressure generator to the pressure fluid conduits leading to the wheel brakes. A system of this type is described in German Patent Application No. p 35 02 018.0.
Brake systems of this type having a hydraulic brake force booster are known in the art. The systems consist of a single-type or tandem-type master cylinder with a hydraulic brake force booster connected upstream thereof, and of auxiliary pressure supply system comprising a hydraulic pump and a hydraulic accumulator. The brake force booster contains an auxiliary pressure control valve which upon actuation of the brake pedal, brings about an auxiliary pressure which is proportional to the pedal force and transmitted to the pistons in the master cylinder. Moreover, brake circuits can be directly connected to a booster chamber in the interior of which prevails the controlled dynamic pressure. The boosting factor of the brake system is selected by the ratio of the surfaces of a transmission piston in the interior of the brake force booster in relation to the surface of an actuating piston mechanically coupled to the brake pedal. As the brake circuits connected to the master cylinder are designed as static circuits, the volume of the pressure chambers in the master cylinder must be adapted to the respective brake system.
Furthermore, slip-controlled brake systems are known in the art comprising a master cylinder including a hydraulic brake force booster connected upstream thereof. In this system, upon commencement of the slip control, dynamic pressure out of the auxiliary pressure supply system will be introduced, by way of the booster chamber, into the static brake circuits connected to the master cylinder thereby compensating the discharge of pressure fluid into the pressure supply reservoir caused to attain pressure reduction during slip control. Brake systems of this type are rather complex and costly and are described in DE-OS 30 40 561 and DE-OS 30 40 562.
In other types of conventional slip-controlled brake systems comprising a master cylinder having a hydraulic brake force booster connected upstream thereof, upon commencement of the slip control, the pressure fluid conduits between the master cylinder and the wheel brakes are interrupted and the auxiliary pressure source, in place of the master cylinder, by way of the brake force booster, is connected to the wheel brakes. During normal braking operations (i.e., until commencement of the slip control) the circuits connected to the master cylinder, are also strictly static brake circuits.
As compared therewith, the brake system as described in the aforementioned patent provides for strictly dynamic braking as long as the auxiliary pressure supply system is intact. The master cylinder only serves to control the auxiliary pressure control valve and to safeguard the brake function upon occurrence of a defective condition (i.e., upon failure of the auxiliary-pressure source) thereby permitting a comparatively simple construction of the overall brake system. In addition to a master cyinder, an auxiliary pressure source and a control valve, only a small number of multidirectional valves will be needed that are pressure controlled and, hence, can be manufactured at low cost. Moreover, it is advantageous that one master cylinder size can be used for differently dimensioned brake systems because the master cylinder only serves to control the pressure in the dynamic brake circuits.
It is, therefore, the object of the invention to enhance the reliability of the brake system as described and, in particular, safeguard that malfunction in the auxiliary pressure supply system, defective conditions of the pressure controlled valves or the like defects are signalled without delay to enable the driver to adapt himself to the changed situation.